A transparent conductive film of tin-doped indium oxide (ITO) has been used widely for display panels such as liquid crystal displays, plasma displays, and electroluminescent (EL) displays, touch panels, and electronic devices such as solar cells. Conductive films including the transparent conductive film have been used by being formed into desired patterns. As a pattern formation method, a chemical etching method using a photoresist or an etching liquid has generally been used. However, there are problems that the number of steps is large, the etching precision may be lowered due to expansion of a resist in a solution, and handling of an etching liquid or waste solution treatment is problematic. As a method for solving the above-mentioned problems, known is a laser abrasion method of radiating near infrared region (NIR) laser light to the conductive film and thereby removing unnecessary parts. Although it is possible to carry out highly precise patterning with no need for a resist, usable base materials are limited, the cost is high, and the treatment speed is low and therefore, it is unsuitable for processing of a conductive film having a large surface area.
On the other hand, as a chemical etching method which does not require the resist step, there is proposed a method of applying a composition containing iron(III) chloride- or iron(III) chloride hexahydrate and a solvent to a region on a surface of an inorganic glass-like crystal to be etched and removing the composition by washing or burning (e.g., refer to Patent Document 2). Further, an etching medium containing phosphoric acid is also proposed (e.g., refer to Patent Document 3). Use of phosphoric acid makes control of the etching speed easy as compared with the case of iron(III) chloride. However, use of any one of these techniques has a problem that the etching does not proceed evenly in a plane and uneven etching occurs.
Furthermore, another method proposed is a method regarding a separation technique of a conductive film containing carbon nanotubes (CNTs) which includes immersing a substrate coated with CNTs in a separation solution, optionally mechanically or chemically stirring the substrate coated with CNTs, and washing the substrate coated with CNTs with a liquid for at least partially removing the CNTs (e.g., refer to Patent Document 4). However, this method is slow in the treatment speed and thus unsuitable for processing of a large surface area and has a problem that the etching does not proceed evenly in a plane and uneven etching occurs.